THE APPLICATION NOTEBOOK – FEBRUARY 2008
General
ADVERTISING SUPPLEMENT
61
Is Bottled Nitrogen a Greenhouse Gas?
Parker Balston
High purity nitrogen gas can be obtained using an “inhouse” system with a hollow fiber membrane which
separates the nitrogen from oxygen and water vapor in
compressed air. A small compressor is used which
requires considerably less energy than fractional distillation of air and transporting tanks to the end-user’s
facility. In addition to reducing the energy requirements
and contribution to greenhouse gases, the use of an
“in-house” system is a safer, more convenient and less
expensive approach than the use of bottled nitrogen.
T
he generation of nitrogen gas for the laboratory provides an
excellent example of how a significant reduction in the generation of greenhouse gases can be obtained by selecting an alternative process that consumes less energy. High purity nitrogen
gas is commonly obtained by the fractional distillation of air,
which involves compression, chilling, re-vaporization, bottling
and transporting to the end-user site. A hollow fiber membrane
can generate nitrogen “in-house” with an high degree of purity
and a considerably smaller energy input.
Nitrogen Generation using Hollow Fiber
Membranes
A hollow fiber membrane preferentially allows oxygen and water
vapor to permeate a membrane wall (Figure 1) while nitrogen
travels through the membrane. “In-house” generation of nitrogen involves:
● Withdrawing air from the atmosphere using a compressor.
● Removing of water vapor/particulates.
● Removing hydrocarbons via an activated carbon filter
● Separating the oxygen and remaining water vapor via hollow
fiber membranes
● Filtering the gas to provide a clean supply of high purity
nitrogen
● Porting the gas to the end device (e.g. LC-MS)
A typical system (Parker Balston N2-14 Nitrogen Generator)
contains thousands of membrane fibers to provide >99.5% N2 at
flow rates up to 467 L/min at 100 psig. The schematic of a system
is shown in Figure 2. In addition, membranes used to dry air and a
nitrogen generating system can be combined to supply individual
streams of dry air, source exhaust air and nitrogen (e.g. for an
LC/MS system).
Energy Considerations for Obtaining Nitrogen via
Fractional Distillation of Air
Obtaining nitrogen using fractional distillation and transport of
the bottles is energy intensive. For example, if 100 bottles are
delivered by a truck that travels 50 miles and gets 10 miles/gallon, 0.05 gallons of fuel/bottle is required and 0.05 gallons to
return bottles, a total of 0.1 gal/bottle.
Nitrogen
Oxygen
Water vapor
Nitrogen
Oxygen and water vapor are “fast” gases
which quickly permeate the membrane,
allowing nitrogen to flow through the
fiber bores as the product stream.
Figure 1: Hollow Fiber Membrane Bundle Separates Nitrogen from Air.
Activated
carbon
filter
Second stage
grade BXE
prefilter
First stage
grade DXE
prefilter
Inlet air gate
valve (customer
supplied)
Operating
pressure
gage
Nitrogen flow
control valve
Nitrogen
flowmeter
Outlet
pressure gage
Final
membrane
filter
To
process
Compressed
air supply
Recommended
upstream
pressure
regulator
(customer
supplied)
Outlet pressure
regulator
Drains
Nitrogen
generator
membrane
module
Figure 2: Schematic of Parker Balston N2-14 Nitrogen Generator
Energy Considerations for Obtaining Nitrogen via
Hollow Fiber Membranes
When a hollow fiber membrane system generates nitrogen, energy is needed for an air compressor. If a 3 hp compressor is
employed with a 50% duty cycle, 25 kWH are used to generate
the equivalent of five 9000 liter gas tanks/day.
Comparing the Greenhouse Gas Contribution with
Fractional Distillation and a Hollow Fiber Membrane
Since more energy is employed for fractional distillation and transport of bottles than using hollow fiber membranes, more greenhouse gases are generated. In house generation of nitrogen gas
using hollow fiber membrane bundles is an effective, environmentally preferred approach to providing pure, clean, dry nitrogen gas.
Parker Hannifin Corporation
Filtration and Separation Division
242 Neck Rd.
Haverhill MA 01835-0723
(800) 343-4048; Fax (978) 556-7503
www.parker.com/ags